1. Field of the Invention
This invention relates to a key telephone system and more particularly to a circuit for controlling a line circuit in a key telephone system.
2. Description of the Prior Art
A key telephone system generally comprises a plurality of key telephone sets, a service unit provided with a line circuit and a common line circuit. A key telephone system employs service features for incoming calls such as line holding, transfer and intercom using locking-type keys in the form of a group of manually operable switches having a common latch arrangement for maintaining one of the switches in the actuated condition until another switch is actuated. The status of the service features is indicated by a visual indicator comprising lamps or light emitting diodes (LEDs), buzzers and the like. Generally, incoming call indication is performed by an intermittent audible tone of a buzzer in synchronism with lighting or extinguishment of the visual indicator representing the presence of an incoming call. Line holding indication is performed by lighting or extinguishment of the visual indicator at a cycle time different from that of an incoming call indicator. Talking indication, representing the in-use status of a telephone line (i.e., "busy" status) is performed by continuous illumination of the visual indicator. Conventionally in order to satisfy these service features, four lines are required, as shown in U.S. Pat. No. Re 26,722. For example line No. 1 in that patent is connected between each line circuit of the service unit and the key telephone sets. Two speech lines T and R, one visual control line LL and one holding control line AL are also provided between each line circuit of the service unit and the key telephone sets.
However, modern key telephone systems are becoming large in size, causing increases in the number of corelines required in cables connected between the service unit and key telephone sets and in the number of corelines in telephone set and terminal connector cables which increases the cost of installation and parts for connection. Furthermore, the increase in the number of corelines and connection parts raises the risk of equipment failure and maintenance difficulties in key telephone systems.